Detection of target molecules or analytes is a key to understanding and controlling complex biological processes such as organismal growth, metabolism, differentiation, cell- and life cycle progression, disease or death. Key requirements of analyte detection are specificity and sensitivity, particularly when the target molecule or analyte is in a limiting amount or concentration in a biological sample.
Typically, specificity is provided by monoclonal antibodies which specifically bind the analyte. Sensitivity is typically provided by a label bound to the specific antibody, or to a secondary antibody which assists detection of relatively low levels of analyte. This type of diagnostic approach has become well known and widely used in the enzyme-linked immunosorbent sandwich assay (ELISA) format. In some cases, enzyme amplification can even further improve sensitivity such as by using a product of a proenzyme cleavage reaction catalyzing the same reaction. Some examples of such “autocatalytic” enzymes are trypsinogen, pepsinogen, or the blood coagulation factor XII. However, in relation to specificity antibodies are relatively expensive and can be difficult to produce with sufficient specificity for some analytes. Polyclonal antibodies also suffer from the same shortcomings and are even more difficult to produce and purify on a large scale.
Current methods to detect specific target molecules and analytes for either prognostic or diagnostic purposes suffer from a number of limitations which significantly restrict their widespread application in clinical, peri-operative and point-of-care settings. Most importantly, the vast majority of diagnostic assays require a significant level of technical expertise and a panel of expensive and specific reagents (most notably monoclonal antibodies) along with elaborate biomedical infrastructures which are rarely available outside specialized laboratory environments. For instance, ELISAs—the gold standard for detecting specific analytes in complex biological samples—rely on the selective capture of a target analyte on a solid surface which in turn is detected with a second affinity reagent that is specific for the target analyte. ELISAs also feature extensive incubation and washing steps which are generally time consuming and difficult to standardize as the number of successive steps frequently introduces significant variation across different procedures, operators and laboratories making quantitative comparisons difficult.